Python: Writing to a file and reading it back out

Question

I am saving a dictionary of student names as keys and grades lists as values. I am attempting to write the values to a file. At the moment I am writing them as strings.

def save_records(students, filename): #saves student records to a file out_file = open(filename, "w") for x in students.keys(): out_file.write(x + " " + str(students[x]) + "\n") out_file.close()

After saving the file, I try to read it back. The pertinent part of the read out is below.

while True: in_line = in_file.readline() if not in_line: break #deletes line read in in_line = in_line[:-1] #initialize grades list in_line = in_line.split() name = in_line[0] students[name] = map(int, in_line[1:])

The read out code works well for normal text files that are pre-formatted. The format of the textfile is: key (whitespace) values separated by whitespace "\n". I would like to know how to write in to a text file by combining string and list elements.

I'm not sure what you mean by "I would like to know how to write in to a text file by combining string and list elements." Can you explain? It seems like your algorithm is working but you want to make it "better". Am I correct? — SethMMorton
– SethMMorton, Commented Feb 8, 2014 at 20:17

kemitche · Accepted Answer · 2014-02-08 20:14:23Z

If you have control over writing the data, I would recommend using a well-established format, such as JSON or INI. This would allow you to make use of common libraries, such as the json or ConfigParser modules, respectively.

beoliver · Accepted Answer · 2014-02-08 20:20:30Z

Would it not be easier to use something like pythons pickle which is for storing things like dicts

...and then pretty print output to a separate file?

It's hard to say without knowing how you plan on using this...

pink vertex · Accepted Answer · 2014-02-08 20:30:25Z

Since students[name] = map(int, in_line[1:]), i assume you want to print the items of the list student[x] with whitespaces inbetween.

You could use the str.join method

' '.join(map(str,students[x]))

superjump · Accepted Answer · 2014-02-08 20:34:52Z

You may want to consider using Comma Separated Value files (aka csv files) instead of plain text files, as these provide a more structured way to read and write your data. Once written, you can open them in a spreadsheet program like Excel to view and edit their contents.

Re-writing your functions to work with csv files, and assuming you are using Python 2.x, we get something like:

import csv def save_records(students, filename): # note that csv files are binary so on Windows you # must write in 'wb' mode; also note the use of `with` # which ensures the file is closed once the block is # exited. with open(filename, 'wb') as f: # create a csv.writer object csv_out = csv.writer(f) for name, grades in students.iteritems(): # write a single data row to the file csv_out.writerow([name]+grades) def read_records(filename): students = dict() # note that we must use 'rb' to read in binary mode with open(filename, 'rb') as f: # create a csv.reader object csv_in = csv.reader(f) for line in csv_in: # name will have type `str` name = line[0] grades = [int(x) for x in line[1:]] # update the `students` dictionary students[name] = grades return students

Wouldn't it be easier to use the csv module in the python standard library to read and write .csv files?
@SethMMorton: that's what I've used, but you may have read my post mid-edit, before I put in the import statement?
Yup, that must have been what happened, because when I saw it, it wasn't there.

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